Water supplies contaminated with arsenic (As) are a major health and environmental concern in the United States and worldwide. Arsenic is a naturally occurring element that is present in rocks or soils. Over time, the arsenic leaches from the rocks or soils into groundwater, surface water, wells, or other sources of drinking water. This arsenic contamination is referred to as indigenous arsenic contamination. Arsenic-contaminated solutions are also produced in a variety of industries, such as mining, agriculture, semiconductor, or petroleum industries. These arsenic-contaminated solutions include process solutions and waste streams. The ingestion of significant amounts of arsenic can lead to disastrous effects on human health, which has become well documented in parts of the Indian subcontinent, where tube wells have replaced surface water as the primary drinking water source. Many of the worst cases of arsenic poisoning have occurred in the West Bengal area, where As concentrations in the drinking water can exceed 300 parts per billion (ppb). Health problems due to the consumption of As-contaminated water currently affect over 70 million people in Bangladesh. However, the deleterious consequences of lesser concentrations of As in drinking water are becoming evident in other parts of the world as well, e.g., Mexico, Argentina, and Taiwan. In September 2001, a National Academy of Science study concluded that even trace amounts of arsenic can cause bladder and lung cancer.
Arsenic is present in nature in valence states or oxidation states of +3 and +5. In water supplies, arsenic contaminants typically exist as As(III) compounds and/or As(V) compounds. The As(III) compounds include As(III) oxyanions or oxyacids, such as H3AsO3 or H2AsO31−, depending on the pH of the water supply. The As(V) compounds include As(V) oxyanions, such as H2AsO4− or HAsO42−, or oxyacids, such as H3AsO4, depending on the pH of the water supply. Under atmospheric conditions or an oxidizing environment, As(V) compounds are predominantly present in water supplies. As(III) compounds are also known as arsenites, while As(V) compounds are known as arsenates.
Numerous techniques for removing arsenic from water supplies have been proposed and developed. For instance, arsenic removal has utilized anion exchange, cation exchange, polymeric anion exchange, liquid-liquid extraction, activated alumina sorption, coprecipitation, sorption by iron oxide-coated sand particles, enhanced coagulation with alum or ferric chloride dosage, ferric chloride coagulation followed by microfiltration, pressurized granulated iron particles, iron oxide doped alginate, manganese dioxide-coated sand, polymeric ligand exchange, and zero-valent iron. These techniques primarily rely on ion exchange and Lewis acid-base interactions to remove the arsenic.
In U.S. Pat. No. 5,591,346 to Etzel et al., an iron(III)-complexed cation exchange resin is disclosed for removing arsenic from wastewater or drinking water. The iron(III)-complexed cation exchange resin is formed by loading a strong acid cation exchange resin with iron ions. The cation exchange resin is purchased commercially and then loaded with the iron ions. When the iron(III)-complexed cation exchange resin is contacted with a stream of wastewater or drinking water, the iron ions react with arsenate anions to form an iron arsenate salt complex. The iron arsenate salt complex is immobilized on the cation exchange resin, removing the arsenic from the wastewater or drinking water.
While many techniques for removing arsenic from water supplies are known, conventional ion exchange resins do not provide the specificity to economically remove low concentrations of arsenic. Since many water supplies in the United States, such as groundwater, surface water, or wells, have low concentrations of arsenic, these techniques are not effective to remove the arsenic. In addition, many of these techniques are not selective for arsenic over other ions. To improve the selectively of ion exchange resins for arsenic, granules of metal oxides or metal hydroxides, such as ferric hydroxide, have also been investigated. While these metal oxide or metal hydroxide granules are more selective for arsenic, they have a low porosity and, therefore, have a low capacity for arsenic and poor kinetic properties. To improve the performance of ferric hydroxide, ferric hydroxide has been incorporated into organic polymers. For instance, in “Arsenic Removal Using a Polymeric/Inorganic Hybrid Sorbent,” DeMarco et al., Water Research 37 (2003) pp. 164-176, a hydrated iron oxide is dispersed into a polymeric, cation exchange bead. The polymeric, cation exchange beads are commercially available and include a polystyrene matrix having sulfonic acid functional groups. A sorbent is prepared by loading Fe(III) onto the sulfonic acid sites on the cation exchange beads. The Fe(III) is then desorbed and Fe(III) hydroxides are simultaneously precipitated within the cation exchange beads using a strong alkaline solution, encapsulating the hydrated iron oxides within the cation exchange beads. The capacity of the sorbent for arsenic is limited by the total number of sulfonic acid sites on the cation exchange beads. In this sorbent, the hydrated iron oxide is loaded at approximately 0.9% to 1.2% by mass. In other words, only 9 mg of iron per gram of sorbent is loaded at saturation.
U.S. Pat. No. 7,368,412 to Tranter et al. (“the '412 patent”) and U.S. Patent Application Publication No. 20050288181 to Tranter et al. (“the '181 application”) describe adsorption media that are used to remove a constituent, such as arsenic, selenium, or antimony, from a feed stream. In one embodiment, the adsorption media include polyacrylonitrile (PAN) and a metal hydroxide or metal oxide as an active component. The active component is ferric hydroxide or hydrated iron oxide (HIO). To synthesize HIO particles, an iron(III) salt is dissolved in water to form a solution. The solution is then titrated to a pH of approximately 6.5 with an aqueous caustic solution to precipitate brown HIO particles, which are recovered by filtration and dried. Since water is used as the reaction medium, the resulting HIO particles include greater than or equal to approximately 70% by mass of water. The HIO particles are also washed with water to remove metal salts produced during the synthesis. However, before the HIO particles are incorporated into the PAN, the water is removed. Drying techniques capable of removing such large amounts of water are expensive due to the large capital and operational costs. In addition, the energy requirements for the drying techniques are expected to be expensive. After drying, the HIO particles are suspended in dimethylsulfoxide (DMSO) and the resulting suspension is combined with PAN, which is soluble in DMSO. The PAN/HIO solution/suspension is sprayed into an aqueous quenching bath, producing the adsorption media in the form of beads that contain HIO and PAN. The beads are removed from the quenching bath, rinsed with water, and dried before use. Multiple washes are conducted to remove residual DMSO and metal salts produced during the formation of the adsorption media. If the washes are not conducted, the surface area and efficiency of the adsorption media are reduced.
While the '412 patent and the '181 application describe effective methods of forming metal oxides, it would be desirable to form metal oxides having a relatively lower water content.